The present invention relates to the field of running shoes and, in particular, to running shoe soles which are formed of a lightweight, multilayered or otherwise non-homogeneous construction. The present invention also relates to the problem of excess "pronation".
As described in detail in my book entitled The Running Shoe Book, Anderson World, Inc., 1980, during running, initial contact between a runner's shoe and the ground occurs at the outside or lateral edge of the shoe and not the back edge thereof, as occurs during walking (see upper illustration, FIG. 4). After landing on the lateral border of the shoe, the foot and the shoe tend to shift quickly into a flat position (central illustration, FIG. 4). This flattening out of the foot involves the subtalar joint (which is the joint between the talus and the heel bone). From the flat position, this lateral rolling motion then continues into the condition known as "pronation" (lower illustration, FIG. 4), wherein the foot is angled inwardly upon its inside edge. This lateral rolling movement into pronation only causes trouble when pronation does not stop within what is considered a normal range. Since pronation involves a rotation at the subtalar joint, it involves both the leg and the foot, so that as the subtalar joint pronates, the leg rotates inward. If there is too much pronation, a large amount of inward rotation occurs which produces a screwing type of motion at the knee joint. This screwing type of motion is one that the knee is not designed to resist, so that, when excessive amounts of prontation occur, the runner's knee joint is likely to be injured.
A very old technique for correction of pronation is known as as the "varus wedge" and is designed so that the platform where the foot is placed is higher on the inside of the shoe than the outside. This is accomplished by tapering the midsole (at an angle usually of between 4.degree. and 8.degree.), so that it is thicker on the inside border. This turns the rear foot in toward the midline of the body, so as to bring the subtalar joint into a neutral position. However, since all runners do not pronate to the same degree, and some do not pronate at all, some podiatrists feel that a varus wedge should be prescribed on a custom basis by a doctor and not utilized as a "standardized wedge" in commercially mass-produced shoes. For example, use of a shoe with a varus wedge by a runner who does not pronate could cause the runner to literally fall off the outside of his shoe, and may increase his risk of both inversion sprains of the ankle and knee pain.
In the area of running shoes, especially those used for competition, great emphasis has been placed on reducing the weight of the running shoe. Additionally, numerous techniques have been developed for increasing the cushioning provided for the foot during initial contact of the shoe with the ground.
U.S. Pat. No. 4,235,026 discloses an elastomeric shoe sole wherein the heel of the outsole is structured to yield a greater amount at the outer side than at the inner side thereof for the purpose of reducing the effect of the impact on this portion of the bottom of the foot which is repeatedly subjected to high impacts during running. This greater yieldability is achieved by a plurality of openings which extend from the outer side of the sole transversely through the heel of the sole substantially at right angles to the longitudinal center line to approximately the longitudinal center line of the outsole of the heel. While such a structuring of the heel end of the sole, in accordance with this patent, enables the foot at the heel end to roll about its longitudinal axis through such a distance as to reduce the force of impact by slowing the deceleration of the foot, thereby cushioning the foot to an acceptable amount, since the imperforate region of the heel of the sole is an elastomeric as utilized conventionally in athletic shoes (i.e., is a resilient cushioning material), it offers no pronation limiting capabilities, particularly in view of the increased rolling action produced by increasing the yieldability of the outer side of the heel.
U.S. Pat. No. 4,128,950 discloses an athletic shoe having a multilayered sole with an improved foam midsole. The multilayered sole of this patent includes an outersole layer of hard resilient wear-resistant rubber material, an intermediate sole layer of resilient lightweight synthetic plastic foam cushioning material, and a heel sole layer of similar material disposed between the intermediate sole layer and the outer sole. In comparison to a standard crepe rubber as used in athletic shoe sole layers (which has a hardness durometer of 44-46), plastic foam materials described in this patent have a hardness durometer of 26-37. While such synthetic plastic foams are very lightweight, their compliancy can allow the wearer's foot to twist when it strikes the ground, so that this patent recommends use of a stabilizer to increase lateral stability of the shoe. More particularly, a thin rectangular stabilizer plate of about 1/16 inch thick semi-rigid, solid synthetic plastic (such as nylon, polyethylene or polypropylene) is located between the intermediate sole layer and the heel sole layer. This stabilizer plate is relatively narrow and either extends fully across the heel or can terminate approximately in a quarter of the distance from the outside of the sole, so as to provide lateral stability on the inside of the sole, but allow some flexibility at the outside thereof. However, due to the fact that the disclosed stabilizer plate is located between two highly compliant foam layers, and is of only a limited front-to-back extent, it can, at best, have only a limited effect on pronation. Furthermore, provision of such a plate increases the manufacturing costs associated with producing the sole. As an alternative stabilizer, this patent also discloses constructing the heel sole layer so as to have an inner core of a lightweight, low density synthetic plastic foam material which is surrounded by an outer border portion of a harder, higher density material, such as a relatively dense, resilient closed cell foam material or a standard crepe rubber. Not only does such an arrangement unnecessarily increase the weight and reduce the compliancy of the sole at the outer border and central rear border of the heel, but, since the harder, border material is merely of the same hardness and resiliency as conventional athletic sole layers, it is no more effective in reducing pronation than a sole having a heel of 100% standard athletic shoe sole layer material.